Host cells such as yeast, filamentous fungi and bacteria have been used to express and secrete proteins that are foreign or heterologous to the hosts. Production of these foreign or heterologous proteins in yeast, filamentous fungi and bacteria involves the expression and partial or complete purification from the host cell. Purification can be greatly simplified if the heterologous proteins are secreted from the cell into the media.
Proteins that are secreted from the cell possess a signal peptide sequence. In the cell, the signal peptide sequence functions to direct a protein into the endoplasmic reticulum (ER). Movement into the lumen of the ER represents the initial step into the secretory pathway of the host cell. Thus, the addition of a signal peptide sequence to a non-secretory protein could promote secretion of that protein.
A number of industrial enzymes (e.g., proteases, amylases, glucoamylases, cellulases, xylanase and phytases) which are produced either endogenously or exogenously in a host cell are secreted into the medium.
Methods for expression of heterologous proteins and their secretion in a biologically active mature form using host cells such as yeast, filamentous fungi and bacteria as the expression system are needed.